The present teachings relate to Roots-type blowers, and more particularly, to such blowers in which the lobes are not straight (e.g., parallel to the axis of the rotor shafts), but instead are “twisted” to define a helix angle.
Roots-type blowers may be used for moving volumes of air in applications such as boosting or supercharging vehicle engines. A Roots-type blower supercharger may be configured to transfer, into the engine combustion chambers, volumes of air which are greater than the displacement of the engine, thereby raising (“boosting”) the air pressure within the combustion chambers to achieve greater engine output horsepower. The present disclosure is not limited to a Roots-type blower for use in engine supercharging, but will be described in connection therewith for illustrative purposes.
In some configurations, a Roots-type blower may include two rotors each having two straight lobes. In other configurations, Roots-type blowers may include three lobes and the lobes may be twisted. In some configurations, a Roots-type blower may include two identical rotors, wherein the rotors may be arranged so that, as viewed from one axial end, the lobes of one rotor are twisted clockwise, while the lobes of the meshing rotor are twisted counterclockwise. Twisted lobes on the rotors of a blower may result in a blower having significantly better air handling characteristics, which may include producing significantly less air pulsation and turbulence.
An example of a Roots-type blower is shown in U.S. Pat. No. 2,654,530, assigned to the assignee of the present invention and incorporated herein by reference in its entirety. Some Roots-type blowers, which may be used as vehicle engine superchargers, may be of a “rear inlet” and/or “axial inlet” type, e.g., a supercharger may mechanically driven by means of a pulley that may be disposed toward the front end of the engine compartment while the air inlet to the blower is disposed at the opposite end, e.g., toward the rearward end of the engine compartment. In some Roots-type blowers, the air outlet may be formed in a housing wall, such that the direction of air flow as it flows through the outlet may be radial relative to the axis of the rotors. Such blowers may be referred to as being of the “axial inlet, radial outlet” type. It should be understood that the present disclosure is not limited to use in the axial inlet, radial outlet type, but will be described in connection therewith for example only.
Another example of a Roots-type blower is shown in U.S. Pat. No. 5,078,583, also assigned to the assignee of the present invention and incorporated herein by reference in its entirety. Roots-type blowers of the “twisted lobe” type may include an outlet port that is generally triangular, and the apex of the triangle may disposed in a plane containing an outlet cusp defined by the overlapping rotor chambers. Typically, angled sides of the triangular outlet port define an angle which is substantially equal to the helix angle of the rotors (e.g., the helix angle at the lobe O.D.), such that each lobe, in its turn, passes by the angled side of the outlet port in a “line-to-line” manner. In accordance with the teachings of the above-incorporated U.S. Pat. No. 5,078,583, some Roots-type blowers include a backflow slot on either side of the outlet port to provide for backflow of outlet air to transfer control volumes of air trapped by adjacent unmeshed lobes of the rotor, just prior to traversal of the angled sides of the outlet port. The present disclosure is not limited to use with a blower housing having a triangular outlet port in which the angle defined by the angled side corresponds to the helix angle of the rotors, but will be described in connection therewith for example only.
Roots-type blowers may include overlapping rotor chambers, with the locations of overlap defining what are typically referred to as a pair of “cusps.” An “inlet cusp” may refer to the cusp adjacent the inlet port and the term “outlet cusp” may refer to the cusp which is interrupted by the outlet port. It should be understood that references to a “helix angle” of the rotor lobes may include the helix angle at the pitch circle of the lobes.
In examples of the present teachings, a Roots-type blower may include a “seal time” wherein the reference to “time” may actually be an angular measurement (e.g., in rotational degrees). Therefore, “seal time” may refer to the number of degrees that a rotor lobe (or a control volume) travels in moving through a particular “phase” of operation, as the various phases will be described hereinafter. In examples of the present teachings, a lobe separation may include the number of degrees between adjacent lobes. In some configurations, for a Roots-type blower having three lobes, the lobe separation (L.S.) may be represented by the equation: L.S.=360/N and with N=3, the lobe separation L.S. may be 120 degrees. A Roots-type blower may include four phases of operation, and for each phase there may be an associated seal time as follows: (1) an “inlet seal time,” which may include the number of degrees of rotation during which the control volume is exposed to the inlet port; (2) a “transfer seal time,” which may include the number of degrees of rotation during which the transfer volume is sealed from both the inlet “event” and the backflow “event”; (3) a “backflow seal time,” which may include the number of degrees during which the transfer volume is open to a backflow port, prior to discharging to the outlet port; and (4) an “outlet seal time,” which may include the number of degrees during which the transfer volume is exposed to the outlet port.
Another parameter of a Roots-type blower may include a twist angle of each lobe (e.g., angular displacement, in degrees), which may occur in “traveling” from the rearward end of the rotor to the forward end of the rotor. In some configurations, a Roots-type blower may include a particular twist angle and that angle may be utilized in designing and developing subsequent blower models. By way of example only, a sixty degree twist angle on the lobes of blower rotors may be employed, and it may correspond to the largest twist angle that a lobe hobbing cutter can accommodate. In examples of the present teachings, the twist angle may be predetermined and the helix angle for the lobe may then be determined, such as described in further detail subsequently. In some configurations, a Roots-type blower may include a greater twist angle (for example, as much as 120 degrees), which may result in a higher/greater helix angle and an improved performance, specifically, a higher thermal compressor efficiency, and lower input power.
In some configurations, air flow characteristics of a Roots-type blower and the speed at which the blower rotors can be rotated may be a function of the lobe geometry, including the helix angle of the lobes. It may be desirable for the linear velocity of the lobe mesh (e.g., the linear velocity of a point at which meshed rotor lobes move out of mesh) to approach the linear velocity of the air entering the rotor chambers through the inlet port. If the linear velocity of the lobe mesh (which may be referred to hereinafter as “V3”) is much greater than the linear velocity of incoming air (which may be referred to hereinafter as “V1”), the movement of the lobe may, in effect, draw at least a partial vacuum on the inlet side. Such a mismatch of V1 and V3 may cause pulsations, turbulence, and/or noise, and creating such requires “work.” Pulsations, turbulence, and/or noise may be may undesirable, such as for an engine supercharger that may rotate at speeds of as much as 15,000 to about 18,000 rpm or more.
It would be desirable to increase the “pressure ratio” of a blower (e.g., the ratio of the outlet pressure (absolute) to inlet pressure (absolute)). A higher pressure ratio may result in a greater horsepower boost for the engine with which the blower is associated. In some configurations, it may be desirable to prevent a Roots-type blower from exceeding a pressure ratio that results in an outlet air temperature in excess of 150 degrees Celsius.